linear no-threshold model (LNT)

The model

The linear no-threshold (LNT) model is a dose-response model used in radiation protection to estimate stochastic health effects such as radiation-induced cancer, genetic mutations and teratogenic effects. It assumes a linear relationship between dose and health effects even at very low doses, where biological effects are harder to observe; it therefore implies that all exposure to ionizing radiation is harmful, however low the dose, and that the effect is cumulative over a lifetime. Stochastic effects occur by chance and their probability, not severity, is proportional to dose. This distinguishes LNT from deterministic effects such as acute radiation syndrome, which reliably occur above a threshold and whose severity rises with dose; LNT is not a model for those.

Regulatory and policy use

Regulators commonly use LNT as a basis for public health policy and dose limits, and the U.S. Nuclear Regulatory Commission (NRC) uses it for regulatory dose limits. In its 2005 report the ICRP concluded that while a low-dose threshold does not seem unlikely for radiation-related cancers of certain tissues, the evidence does not favour a universal threshold, and that the LNT hypothesis, combined with an uncertain DDREF for extrapolation from high doses, remains a prudent basis for protection at low doses and low dose rates. In 2007 the ICRP noted that collective dose is effective for optimization but that aggregating very low doses to estimate excess cancers is inappropriate because of large uncertainties.

Because LNT is used to extrapolate expected extra deaths from environmental radiation exposure, it strongly affects public policy: a radiation release is translated into lives lost, and any reduction in exposure, for example through radon detection, into lives saved. At very low doses the model predicts new cancers only in a very small fraction of the population, but across a large population the number is extrapolated into hundreds or thousands.

In 2025 Donald Trump issued an executive order proposing "determinate radiation limits" to replace the linear no-threshold model and the ALARA principle, aimed at easing licensing requirements on new nuclear power plants in the United States.

Origins

The association of radiation exposure with cancer had been observed as early as 1902, six years after Röntgen's discovery of X-rays and Becquerel's of radioactivity. In 1927 Hermann Muller demonstrated that radiation may cause genetic mutation and suggested mutation as a cause of cancer. Muller, who received a Nobel Prize in 1946 for his work on the mutagenic effect of radiation, asserted in his Nobel lecture that mutation frequency is "directly and simply proportional to the dose of irradiation applied" and that there is "no threshold dose".

Early studies were based on higher radiation levels, which made the safety of low levels hard to establish. Interest intensified after the atomic bombings of Hiroshima and Nagasaki and studies were conducted on survivors, but by the late 1940s, with compelling evidence on low doses hard to come by, LNT became more popular for its mathematical simplicity. In 1954 the National Council on Radiation Protection and Measurements (NCRP) introduced the concept of maximum permissible dose. In 1958 the UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) assessed LNT and a threshold model but noted the difficulty of acquiring reliable information about the correlation between small doses and their effects. In the same period the U.S. Congress Joint Committee on Atomic Energy, unable to establish whether a threshold or "safe" level exists, introduced the concept of "As Low As Reasonably Achievable" (ALARA), which became a fundamental principle of radiation protection policy and implicitly accepted LNT.

In 1959 the U.S. Federal Radiation Council supported LNT extrapolation down to the low dose region in its first report. By the 1970s LNT had become accepted as the standard in radiation protection practice by a number of bodies. In 1972 the first report of the National Academy of Sciences (NAS) Biological Effects of Ionizing Radiation (BEIR) committee supported LNT on pragmatic grounds, noting that the dose-effect relationship may not be linear but that linear extrapolation may be justified as a basis for risk estimation. Its seventh report of 2006 (BEIR VII) concluded that the preponderance of information indicates some risk even at low doses.

Dispute and criticism

LNT is supported by regulatory bodies and scientific organizations, but it has been contested by a number of scientists. The threshold model assumes very small exposures are harmless, and the radiation hormesis model claims radiation at very small doses can be beneficial. Because the current data are inconclusive, scientists disagree on which model to use, though most national and international cancer research organizations explicitly endorse LNT for regulating low-dose exposures.

It has been claimed that Hermann Muller, an early proponent, intentionally ignored an early study that did not support LNT when he advocated the model in his 1946 Nobel lecture. It is also argued that the human body has defence mechanisms such as DNA repair and programmed cell death that protect against carcinogenesis from low-dose exposures, though these repair mechanisms are known to be error prone.

In 2005 the French Academy of Sciences and the National Academy of Medicine published a report rejecting LNT in favour of a threshold dose response and a significantly reduced risk at low exposure. The Health Physics Society's position statement, first adopted in January 1996 and last revised in February 2019, advises against estimating health risks from exposures near or below natural background levels because statistical uncertainties are great. UNSCEAR stated in its 2012 report that it does not recommend multiplying very low doses by large numbers of individuals to estimate numbers of radiation-induced health effects.

In 2021 the NRC upheld LNT, following challenges to its dose limit requirements, as "a sound regulatory basis for minimizing the risk of unnecessary radiation exposure to both members of the public and radiation workers". It concluded that the actual level of risk at low doses remains uncertain and that no national or international authoritative scientific advisory body has concluded that evidence of a threshold exists. The U.S. National Academies' BEIR VII Phase 2 meta-analysis of 2005 stated that the research base shows no threshold of exposure below which low levels of ionizing radiation can be demonstrated to be harmless or beneficial.

Psychological effects and Chernobyl

It has been argued that LNT caused an irrational fear of radiation. In the wake of the 1986 Chernobyl accident, anxieties spread across Europe among pregnant mothers over the perception that their children would be born with a higher rate of mutations, and hundreds of induced abortions of healthy unborn children were performed as far away as Switzerland. A 1999 assessment of data sets approaching a million births, divided into "exposed" and control groups, detected no Chernobyl impacts, and the researchers concluded that in retrospect the widespread fear about possible effects on the unborn was not justified. The consequences of low-level radiation are often more psychological than radiological; forced evacuation may lead to social isolation, anxiety, depression, psychosomatic problems, reckless behaviour or suicide. A comprehensive 2005 study concluded that the mental health impact of Chernobyl is the largest public health problem unleashed by the accident to date.